Engines may be configured with exhaust gas recirculation (EGR) systems to divert at least some exhaust gas from an engine exhaust manifold to an engine intake manifold. By providing a desired engine dilution, such systems reduce engine knock, throttling losses, in-cylinder heat losses, as well as NOx emissions. As a result, fuel economy is improved, especially at higher levels of engine boost. Engines have also been configured with a sole cylinder (or cylinder group) that is dedicated for providing external EGR to other engine cylinders. Therein, all of the exhaust from the dedicated cylinder group is recirculated to the intake manifold. As such, this allows a substantially fixed amount of EGR to be provided to engine cylinders at most operating conditions. By adjusting the fueling of the dedicated EGR cylinder group (e.g., to run rich), the EGR composition can be varied to include species such as Hydrogen which improve the EGR tolerance of the engine and resulting fuel economy benefits.
Engine systems with dedicated EGR cylinder groups may be configured with a diverter valve which allows all the exhaust from the dedicated EGR cylinder group to either be directed back to the intake manifold or diverted to the exhaust catalyst. An example of such a system with a diverter valve is shown by Hayman et al. in U.S. Pat. No. 8,539,768. By diverting exhaust from the dedicated EGR cylinder to the exhaust catalyst, heat flow to the catalyst can be increased, such as during engine cold-start and catalyst warm-up conditions. In addition, the EGR rate can be decreased at light loads.
However, the inventors herein have identified potential issues with such an approach. As an example, the diverter valve may provide limited choices. In particular, the controller may be constrained between directing all the exhaust to the intake manifold, which improves fuel economy but reduces exhaust catalyst temperature, or directing all the exhaust to the catalyst, which improves catalyst temperature but reduces fuel economy. Thus during the redirecting of exhaust for catalyst warming, EGR is temporarily disabled, even though engine dilution may be required. As such, this results in a drop in engine performance and fuel economy. As another example, there may be conditions other than during an engine cold-start when catalyst temperature control is required. For example, the exhaust catalyst can cool below its optimal conversion temperature during extended light load operation since pre-catalyst exhaust temperatures are typically lower at lighter loads. The exhaust gas temperature may be further cooled by the addition of high levels of EGR to the combustion chamber. As such, if the exhaust catalyst temperature drops below a threshold temperature during engine operation, an emissions conversion rate is degraded, adversely affecting engine exhaust emissions.
The inventors have recognized these issues and have developed a method for exhaust catalyst temperature control that at least partly overcomes some of the issues. One example method includes flowing exhaust from a dedicated EGR cylinder to each of an exhaust catalyst via a bypass passage and an engine intake via an EGR passage; and adjusting a relative flow through the passages via a bypass valve, the adjusting responsive to catalyst temperature. In this way, exhaust may be flowed from the dedicated EGR cylinder to each of the engine intake and the exhaust catalyst, concurrently, their relative ratios metered to provide a desired exhaust catalyst temperature.
As an example, an EGR passage coupling a dedicated EGR cylinder to an engine intake may include a continuously variable bypass valve that allows a portion of the exhaust gas to be metered to an exhaust catalyst in the exhaust manifold via a bypass passage. As such, the remaining portion of the exhaust gas may continue to be recirculated to the engine intake via the EGR passage. Based on a temperature of the exhaust catalyst, the bypass valve may be adjusted to vary a ratio of exhaust flow through the bypass passage relative to the EGR passage. For example, during conditions when catalyst temperature is below a threshold, such as during a cold-start condition or after an extended operation at light load, the bypass valve may be adjusted to increase exhaust flow through the bypass passage while correspondingly decreasing exhaust flow through the EGR passage. In addition, the dedicated EGR cylinder may be enriched to provide a hydrogen, CO2 and hydrocarbon-rich exhaust stream at the exhaust catalyst. The degree of richness may be adjusted based on the heat flux required to bring the exhaust catalyst to or above a threshold temperature. For example, for a given flow rate through the bypass passage, as a difference between the exhaust catalyst and the threshold temperature increases, the dedicated EGR cylinder may be fueled more rich. Concurrently, remaining engine cylinders may be fueled lean, the degree of leanness adjusted based on the degree of richness of the dedicated EGR cylinder to maintain an overall tailpipe exhaust air-fuel ratio at or around stoichiometry. The rich exhaust from the dedicated EGR cylinder may then be combined with the lean exhaust from the remaining cylinders to generate a significantly exothermic reaction at the catalyst, further expediting catalyst heating. As the catalyst temperature exceeds the threshold, the bypass valve may be adjusted to reduce exhaust flow through the bypass while increasing exhaust flow recirculated to the engine intake.
In this way, a bypass valve coupled to a dedicated EGR system may be actuated to maintain an exhaust catalyst temperature. By continuously diverting at least some exhaust gas to an exhaust catalyst while recirculating a remaining portion of the exhaust gas to the engine intake, catalyst temperature control is enabled without requiring EGR to be disabled. By enriching the dedicated EGR cylinder group based on the flow through the bypass valve, catalyst heating can be expedited. By combining rich exhaust (which is hydrocarbon-enriched) from the dedicated EGR cylinder group with lean exhaust (which is oxygen-enriched) from remaining engine cylinders at the exhaust catalyst, a significant exothermic reaction can be generated to maintain the exhaust catalyst above an activation temperature. In addition, by enabling the exothermic reaction to occur directly at the exhaust catalyst, heat transfer is improved, and heat loss to other engine components (such as the cylinder head, turbine, exhaust plumbing, etc.) is reduced. By allowing catalyst temperature to be controlled without disabling EGR delivery, exhaust emissions can be improved without incurring fuel economy losses.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.